Some flags are used internally by the allocators for management
purposes. One example of that is the CFLGS_OFF_SLAB flag that slab uses
to mark that the metadata for that cache is stored outside of the slab.

No cache should ever pass those as a creation flags. We can just ignore
this bit if it happens to be passed (such as when duplicating a cache in
the kmem memcg patches).

Because such flags can vary from allocator to allocator, we allow them
to make their own decisions on that, defining SLAB_AVAILABLE_FLAGS with
all flags that are valid at creation time. Allocators that doesn't have
any specific flag requirement should define that to mean all flags.

Common code will mask out all flags not belonging to that set.

[ v2: leave the mask out decision up to the allocators ]
[ v3: define flags for all allocators ]
[ v4: move all definitions to slab.h ]

+ /*
+ * Some allocators will constraint the set of valid flags to a subset
+ * of all flags. We expect them to define CACHE_CREATE_MASK in this
+ * case, and we'll just provide them with a sanitized version of the
+ * passed flags.
+ */
+ flags &= CACHE_CREATE_MASK;

> Some flags are used internally by the allocators for management
> purposes. One example of that is the CFLGS_OFF_SLAB flag that slab uses
> to mark that the metadata for that cache is stored outside of the slab.
>
> No cache should ever pass those as a creation flags. We can just ignore
> this bit if it happens to be passed (such as when duplicating a cache in
> the kmem memcg patches).
>
> Because such flags can vary from allocator to allocator, we allow them
> to make their own decisions on that, defining SLAB_AVAILABLE_FLAGS with
> all flags that are valid at creation time. Allocators that doesn't have
> any specific flag requirement should define that to mean all flags.
>
> Common code will mask out all flags not belonging to that set.
>
> [ v2: leave the mask out decision up to the allocators ]
> [ v3: define flags for all allocators ]
> [ v4: move all definitions to slab.h ]
>
> Signed-off-by: Glauber Costa <glommer@parallels.com>
> Acked-by: Christoph Lameter <cl@linux.com>
> CC: David Rientjes <rientjes@google.com>
> CC: Pekka Enberg <penberg@cs.helsinki.fi>

> Some flags are used internally by the allocators for management
> purposes. One example of that is the CFLGS_OFF_SLAB flag that slab uses
> to mark that the metadata for that cache is stored outside of the slab.
>
> No cache should ever pass those as a creation flags. We can just ignore
> this bit if it happens to be passed (such as when duplicating a cache in
> the kmem memcg patches).

I may be minunderstanding this, but...

If some caller to kmem_cache_create() is passing in bogus flags then
that's a bug, and it is undesirable to hide such a bug in this fashion?

> Because such flags can vary from allocator to allocator, we allow them
> to make their own decisions on that, defining SLAB_AVAILABLE_FLAGS with
> all flags that are valid at creation time. Allocators that doesn't have
> any specific flag requirement should define that to mean all flags.
>
> Common code will mask out all flags not belonging to that set.

On 10/19/2012 02:42 AM, Andrew Morton wrote:
> On Wed, 17 Oct 2012 15:36:51 +0400
> Glauber Costa <glommer@parallels.com> wrote:
>
>> Some flags are used internally by the allocators for management
>> purposes. One example of that is the CFLGS_OFF_SLAB flag that slab uses
>> to mark that the metadata for that cache is stored outside of the slab.
>>
>> No cache should ever pass those as a creation flags. We can just ignore
>> this bit if it happens to be passed (such as when duplicating a cache in
>> the kmem memcg patches).
>
> I may be minunderstanding this, but...
>
> If some caller to kmem_cache_create() is passing in bogus flags then
> that's a bug, and it is undesirable to hide such a bug in this fashion?
>

Not necessarily.

This part is part of the kmemcg-slab series. In that use case, I copy
the flags from the original kmem cache, and create a duplicate. That
duplicate need to have the same flags, but only the creation flags.

We had many attempts to mask it out in different places, and after some
discussion, it seemed best to independently do it from common code in
slab_common.c at creation time. It gets quite independent from the
kmemcg-slab this way, and so I posted independently to reduce my churn

On Thu, Oct 18, 2012 at 12:07 AM, David Rientjes <rientjes@google.com> wrote:
> On Wed, 17 Oct 2012, Glauber Costa wrote:
>
>> Some flags are used internally by the allocators for management
>> purposes. One example of that is the CFLGS_OFF_SLAB flag that slab uses
>> to mark that the metadata for that cache is stored outside of the slab.
>>
>> No cache should ever pass those as a creation flags. We can just ignore
>> this bit if it happens to be passed (such as when duplicating a cache in
>> the kmem memcg patches).
>>
>> Because such flags can vary from allocator to allocator, we allow them
>> to make their own decisions on that, defining SLAB_AVAILABLE_FLAGS with
>> all flags that are valid at creation time. Allocators that doesn't have
>> any specific flag requirement should define that to mean all flags.
>>
>> Common code will mask out all flags not belonging to that set.
>>
>> [ v2: leave the mask out decision up to the allocators ]
>> [ v3: define flags for all allocators ]
>> [ v4: move all definitions to slab.h ]
>>
>> Signed-off-by: Glauber Costa <glommer@parallels.com>
>> Acked-by: Christoph Lameter <cl@linux.com>
>> CC: David Rientjes <rientjes@google.com>
>> CC: Pekka Enberg <penberg@cs.helsinki.fi>
>
> Acked-by: David Rientjes <rientjes@google.com>